Process for high temperature solution polymerization

ABSTRACT

The current invention provides a method of improving the efficiency of one or more heat exchangers used in cooperation with a high temperature solution polymerization process. Addition of surface active agents, such as C 6  to C 22  carboxylic acids, to a two phase liquid-liquid polymer solution downstream of a reactor system and upstream of a heat exchanger system can increase the efficiency of heat exchange by more than 10%.

FIELD OF THE INVENTION

This invention relates to the area of high temperature solutionpolymerization, more specifically to a polymerization process in whichthe efficiency of an associated heat exchanger system is increased.

BACKGROUND OF THE INVENTION

Solution polymerization processes are carried out at temperatures thatare above the melting point of the product polymer. In a typicalprocess, catalyst components, solvent, polymerizable monomers andhydrogen are fed under pressure to one or more stirred tank reactors.Catalyst components may be fed to the reactor as a solution or as aslurry and the temperature of the reactor is controlled by the rate ofcatalyst addition, the temperature of the catalyst feed stream and/orthe use of heat transfer systems. Typical polymerizable monomers forsolution phase polymerization processes include ethylene, styrene,propylene and various dienes.

For ethylene polymerization, reactor temperatures can range from about130° C. to about 250° C. while pressures are generally in the range offrom about 500 to about 4000 psi. Although catalyst residence times aregenerally short (e.g. minutes) due to the harsh reactor conditions, ifdesired, solution polymerization may be operated under a wide range ofprocess conditions that allow tailoring of the product polymer as wellas rapid product swings.

In solution polymerization, product polymer is molten and remainsdissolved in the solvent under reactor conditions, forming a polymersolution. After a selected hold-up time (i.e. catalyst residence time),the polymer solution leaves the reactor as an effluent stream and thepolymerization reaction is quenched, typically with coordinating polarcompounds, to prevent further polymerization. Once quenched, the polymersolution is typically fed to a flash devolatilization system for solventremoval. Flash devolatilization also removes un-reacted monomers fromthe polymer solution.

Under certain conditions of temperature and pressure, the polymersolution can phase separate into two distinct liquid phases, one whichis “lean” in dissolved polymer and one which is “rich” in dissolvedpolymer. Phase separation occurs at the lower critical solutiontemperature (LCST), also known as the “cloud point”. Increasing thetemperature, or decreasing the pressure at the cloud point leads tofurther phase separation. The cloud point is determined in part by thepressure, temperature, solution composition and the solvent used forpolymerization.

It is generally undesirable to have phase separation occur within thepolymerization reactor, and process conditions such as monomerconcentration, temperature and pressure are controlled to avoidliquid-liquid phase separation. For example, the polymerizationtemperature may be kept between the crystallization boundary and theLCST of the polymer solution for a given pressure, solvent and monomerconcentration. However, once the polymer solution leaves the reactor, itmay be beneficial to promote liquid-liquid phase separation as it canfacilitate separation of volatile components from the polymer product.

U.S. Pat. Nos. 3,553,156 and 3,726,843 assigned to du Pont de Nemoursdescribes a process in which the reactor effluent, an elastomericethylene copolymer solution, is induced to undergo a liquid-liquid phaseseparation into “polymer rich” and “polymer lean” fractions through therelease of pressure by use of a pressure let down valve. The two liquidphases are decanted from one another in a settlement chamber and thepolymer rich phase is fed into a low-pressure separator to boil offresidual solvent and un-reacted monomer. The polymer lean phase isrecycled to the reactor. The process substantially reduces the energylost by evaporation of volatiles (i.e. the heat of vaporization) in adevolatilization chamber by separating out the volatiles in a “polymerlean” liquid phase.

In U.S. Pat. No. 4,857,633 assigned to Exxon Research & Engineering, ahigh temperature solution process is described in which a low molecularweight hydrocarbon is added to a polymer solution to facilitate phaseseparation of a polymer solution under certain conditions of temperatureand pressure.

U.S. Pat. No. 6,881,800 assigned to ExxonMobil, discloses a process andapparatus to separate a polymer solution into polymer rich and polymerlean liquid phases prior to devolatilization. The apparatus includes apressure source, a polymerization reactor, a pressure let down device,and a separator downstream of one another respectively. In the process,the high pressure source is used to maintain a single liquid phase inthe polymerization reactor, while the pressure let-down devicefacilitates the formation of a two-phase liquid-liquid system having apolymer rich phase and a polymer lean phase. Separation of these phasesis accomplished by way of a liquid phase separator that feeds thepolymer rich phase to a chamber at lower pressure in order to flash offresidual solvent and un-reacted monomer.

Similarly, U.S. Pat. No. 5,599,885 assigned to Mitsui Petrochemicals,describes a solution polymerization process in which phase separationdownstream of the reactor is used to facilitate polymer isolation. Thereactor effluent is separated into a lower phase that is rich in polymerand an upper phase that is rich in solvent by increasing the temperatureof the polymer solution within a “separation zone”. The temperature israised to more than 180° C. above the upper cloud point temperature ofthe polymer solution. Polymer is recovered from the lower phase, whilethe upper phase is in part recycled to the reactor.

In U.S. Pat. No. 4,444,922, an improved phase separation process isdescribed. Temperatures and pressures are moderated to produce “spinodaldecomposition” driven phase separation as opposed to “nucleation andgrowth” driven phase separation. Spinodal decomposition driven phaseseparation is a form of phase separation that promotes rapidpartitioning and settling of the polymer lean and polymer rich phases.The process facilitates separation of the distinct liquid phases by wayof a liquid-liquid separator or a decanter.

In a typical devolatilization process, the polymer solution (reactoreffluent) is pre-heated in a heat exchanger and then passed into achamber of reduced pressure. Boiling of solvent and un-reacted monomersoccurs and the vapors are sent to a solvent and monomer recovery systemand are recycled back to the reactor. Heating the polymer solutionupstream of the devolatilization system increases the enthalpy of theproduct stream, providing high temperatures to the polymer melt afterdevolatilization. The high temperatures facilitate flow of the polymermelt by reducing its viscosity. The heat exchangers used are mostcommonly shell and tube type heat exchangers and can increase thetemperature of the polymer solution to as high as about 280° C.

U.S. Pat. No. 4,547,473 describes a typical high temperature solutionprocess for the homo- or co-polymerization of ethylene at temperaturesin excess of 150° C. using a titanium based catalyst system. Solvent isremoved using standard flash devolatilization as described in U.S. Pat.No. 5,708,133.

In PCT application, 98/02471 filed by Dow Chemicals, a solutionpolymerization process is described in which a two stagedevolatilization system is used to remove solvent and un-reactedmonomers from an EPDM (ethylene-propylene-diene monomer) polymersolution. In a preferred embodiment a dual reactor system is used inwhich the temperature of the second reactor is between 90° C. and 120°C. For flash devolatilization, the temperature of the reactor effluentis raised to between 210° C. and 250° C. by passage through a heatexchanger prior to entering the flash chamber that is at lower pressure.

U.S. Pat. No. 5,691,445 assigned to Novacor Chemicals describes apolymer solution devolatilization process in which less than 150 ppm ofresidual volatiles is retained in the isolated polymer. In the process,the polymer solution leaves the polymerization reactor and travelsthrough a pre-heat exchanger. The pre-heat exchanger heats the polymersolution to temperatures from about 200° C. to 270° C. to increase thevapor pressure of volatiles and to reduce the polymer solutionviscosity. In a preferred embodiment, a super-critical fluid is added tothe process at a point between the first and second devolatilizationchambers to enhance polymer melt foaming.

The efficiency of a heat exchanger is a major consideration whendetermining the volume of polymer solution that may be adequately heatedby a given heat transfer fluid. The overall amount of heat transferdepends on a number of factors, including but not limited to thematerials used for construction of a heat exchanger, the area of theheat exchange surface (i.e. the number, length and diameter of tubes inthe tube sheet of a shell and tube type heat exchanger), the rate offlow of polymer solution and/or the heat transfer fluid through the tubeand shell sides of the heat exchanger respectively, whether the flowsare parallel counter-current or parallel co-terminus, the nature offluid flow (turbulent or Newtonian), and the nature and composition ofthe exchanging fluids.

Optimization of heat transfer is most commonly addressed though thedesign and construction of the associated heat exchanger equipment. As aresult, significant capital investment may be required for makingsuitable upgrades such as the installation of inserts to increaseturbulent flow within the heat exchanger tubes, the use of larger heatexchangers or the use of heat exchangers with more heat exchangecapacity. Alternatively, the heat transfer fluid may be heated to highertemperatures, but this requires significantly higher energy input.

There remains a need for improving the efficiency of heat transferwithin the one or more heat exchangers, associated with a solutionpolymerization process, without requiring large capital investments orincreased operating costs.

SUMMARY OF THE INVENTION

The invention provides a practical low cost method of enhancing the heatexchange capacity of a heat exchanger system used in a high temperaturesolution polymerization process.

The current invention provides an improved process for solutionpolymerization in which surface active agents are added to a two phaseliquid-liquid polymer solution to improve the efficiency of heattransfer in the associated heat exchanger system.

In an embodiment of the invention, the heat exchanger system comprisesone or more shell and tube type heat exchangers.

The process of the invention allows for higher flow rates of polymersolution through a heat exchanger system and mitigates the requirementfor higher energy requirements or costly and time consuming upgrades toa heat exchanger.

The present invention provides an improved high temperature solutionpolymerization process, the improvement of which comprises increasingthe heat transfer coefficient, U, of at least one heat exchanger by: (a)inducing a single phase polymer solution to undergo phase separationinto a polymer lean phase and a polymer rich phase; and (b) adding asurface active agent compound.

The present invention provides a high temperature solutionpolymerization process comprising:

(a) polymerizing one or more than one olefin in a solvent within areactor system to produce a single phase polymer solution;

(b) quenching the polymerization reaction downstream of the reactorsystem with a catalyst deactivator;

(c) reducing the pressure of the single phase polymer solutiondownstream of the reactor system to a pressure which induces the singlephase polymer solution to undergo liquid-liquid phase separation into atwo phase polymer solution; and

(d) feeding the two phase polymer solution through a heat exchangersystem,

wherein the heat transfer coefficient, U, of at least one heat exchangeris increased by adding at least one surface active agent to the polymersolution downstream of the reactor system and upstream of the heatexchanger system.

In the present invention, very low levels of surface active agent areadded to a two phase liquid-liquid polymer solution to increase the heattransfer coefficient, U, of a at least one heat exchanger, by more than10%.

In an embodiment of the invention, the surface active agents will beadded in amounts from 0.1 ppm to 1000 ppm, preferably from 1 ppm to 1000ppm, even more preferably from 1 ppm to 100 ppm.

In an embodiment of the current invention, the surface active agent isselected from the group consisting of carboxylate, sulfate, phosphate,phosphonate, and sulfonate compounds comprising a branched orun-branched, saturated or unsaturated alkyl group comprising 6 to 30carbon atom, and mixtures thereof.

In an embodiment of the invention, the pressure of the single phasepolymer solution is reduced by opening one or more pressure let downvalves downstream of the reactor system.

The invention also provides, a high temperature solution polymerizationprocess comprising:

(a) polymerizing one or more than one olefin in a solvent within areactor system to produce a single phase polymer solution;

(b) quenching the polymerization reaction downstream of the reactorsystem with a catalyst deactivator;

(c) reducing the pressure of the single phase polymer solutiondownstream of the reactor system to a pressure which induces the singlephase polymer solution to undergo liquid-liquid phase separation into atwo phase polymer solution; and

(d) feeding the two phase polymer solution through a heat exchangersystem,

wherein the heat transfer coefficient, U, of at least one heat exchangeris increased by adding at least one surface active agent to the polymersolution downstream of the reactor system and upstream of the heatexchanger system, and wherein the surface active agent and the catalystdeactivator are the same compound, provided that the surface activeagent is added in an amount higher than the amount of surface activeagent required to quench the polymerization reaction.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a vapor-liquid-liquid (VLL) phase diagram that shows the phasebehavior of a polydisperse polyethylene polymer solution with a smallamount of ethylene present.

FIG. 2 shows the effect of a surface active agent on the heat transfercoefficient, U, of a heat exchanger that has a polymer solution flowingthrough it. The effect of added surface active agent on the heattransfer coefficient, U, is shown for a given polymer solution at twodifferent pressures.

DETAILED DESCRIPTION

Solution processes for the homo-polymerization or co-polymerization ofethylene are well known in the art. Solution polymerization processesare used commercially to prepare a wide variety of ethylene polymers,ranging from crystalline polyethylene plastics to amorphousethylene-propylene elastomers. It is desirable to operate theseprocesses at high temperatures because increasing the polymerizationtemperature can (a) improve the rate of polymerization; (b) lower theviscosity of the polymer solution; and (c) reduce the amount of energyrequired to recover the polymer from the solvent.

Solution processes are commonly conducted in the presence of an inerthydrocarbon solvent, typically a C₅₋₁₂ hydrocarbon, which may beunsubstituted or substituted by a C₁₋₄ alkyl group, such as pentane,methyl pentane, hexane, heptane, octane, cyclohexane, methylcyclohexaneand hydrogenated naphtha. An example of a suitable solvent that iscommercially available is “Isopar E” (C₈₋₁₂ aliphatic solvent, ExxonChemical Co.).

The polymerization temperature in a high temperature solution process isfrom about 80° C. to about 300° C., preferably from about 120° C. to250° C. The upper temperature limit will be influenced by considerationsthat are well known to those skilled in the art, such as a desire tomaximize operating temperature (so as to reduce solution viscosity),while still maintaining good polymer properties (as increasedpolymerization temperatures generally reduce the molecular weight of thepolymer). In general, the upper polymerization temperature willpreferably be between 200 and 300° C. The most preferred reactionprocess is a “medium pressure process”, meaning that the pressure in thereactor is preferably less than about 6,000 psi (about 42,000kiloPascals or kPa). Preferred pressures are from 10,000 to 40,000 kPa,most preferably from about 2,000 psi to 3,000 psi (about 14,000-22,000kPa).

The pressure in the reactor system should be high enough to maintain thepolymerization solution as a single phase polymerization solution and toprovide the necessary upstream pressure to feed the polymer solutionfrom the reactor system through a heat exchanger system and to adevolatilization system.

Suitable monomers for co-polymerization with ethylene include C₃₋₂₀mono- and di-olefins. Preferred comonomers include C₃₋₁₂ alpha olefinswhich are unsubstituted or substituted by up to two C₁₋₆ alkyl radicals;C₈₋₁₂ vinyl aromatic monomers which are unsubstituted or substituted byup to two substituents selected from the group consisting of C₁₋₄ alkylradicals; C₄₋₁₂ straight chained or cyclic diolefins which areunsubstituted or substituted by a C₁₋₄ alkyl radical. Illustrativenon-limiting examples of such alpha-olefins are one or more ofpropylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, butadiene,styrene, alpha methyl styrene, and the constrained-ring cyclic olefinssuch as cyclobutene, cyclopentene, dicyclopentadiene, norbornene,alkyl-substituted norbornenes, alkenyl-substituted norbornenes and thelike (e.g. 5-methylene-2-norbornene and 5-ethylidene-2-norbornene,bicyclo-(2,2,1)-hepta-2,5-diene).

The polyethylene polymers which may be prepared in accordance with thepresent invention are linear low density polyethylenes (LLDPE's) whichtypically comprise not less than 60, preferably not less than 75 weight% of ethylene and the balance one or more C₄₋₁₀ alpha olefins,preferably selected from the group consisting of 1-butene, 1-hexene and1-octene. The polyethylene prepared in accordance with the presentinvention may be LLDPE having a density from about 0.910 to 0.935 g/ccor (linear) high density polyethylene having a density above 0.935 g/cc.The present invention might also be useful to prepare polyethylenehaving a density below 0.910 g/cc (the so-called very low and ultra lowdensity polyethylenes).

The present invention may also be used to prepare co- and ter-polymersof ethylene, propylene and optionally one or more diene monomers.Generally, such polymers will contain about 50 to about 75 weight %ethylene, preferably about 50 to 60 weight % ethylene andcorrespondingly from 50 to 25 weight % of propylene. A portion of themonomers, typically the propylene monomer, may be replaced by aconjugated diolefin. The diolefin may be present in amounts up to 10weight % of the polymer although typically is present in amounts fromabout 3 to 5 weight %. The resulting polymer may have a compositioncomprising from 40 to 75 weight % of ethylene, from 50 to 15 weight % ofpropylene and up to 10 weight % of a diene monomer to provide 100 weight% of the polymer. Preferred but not limiting examples of the dienes aredicyclopentadiene, 1,4-hexadiene, 5-methylene-2-norbornene,5-ethylidene-2-norbornene and 5-vinyl-2-norbornene, especially5-ethylidene-2-norbornene and 1,4-hexadiene.

The solution polymerization process of this invention uses a stirred“reactor system” comprising one or more stirred polymerization reactors.In a dual reactor system, the first polymerization reactor preferablyoperates at lower temperature. The residence time in each reactor willdepend on the design and the capacity of the reactor. Generally thereactors should be operated under conditions to achieve a thoroughmixing of the reactants. In addition, it is preferred that from 20 to 60weight % of the final polymer is polymerized in the first reactor, withthe balance being polymerized in the second reactor. On leaving thereactor system the polymer solution is passed through a heat exchangersystem on route to a devolatilization system and polymer finishing areaas described further below.

The monomers are dissolved/dispersed in the solvent either prior tobeing fed to the first reactor (or for gaseous monomers the monomer maybe fed to the reactor so that it will dissolve in the reaction mixture).Prior to mixing, the solvent and monomers are generally purified toremove potential catalyst poisons such as water, oxygen or metalimpurities. The feedstock purification follows standard practices in theart, e.g. molecular sieves, alumina beds and oxygen removal catalystsare used for the purification of monomers. The solvent itself as well(e.g. methyl pentane, cyclohexane, hexane or toluene) is preferablytreated in a similar manner. The feedstock may be heated or cooled priorto feeding to the first reactor. Additional monomers and solvent may beadded to the second reactor, and it may be heated or cooled.

Generally, the catalyst components may be premixed in the solvent forthe reaction or fed as separate streams to each reactor. In someinstances premixing it may be desirable to provide a reaction time forthe catalyst components prior to entering the reaction. Such an “in linemixing” technique is described in a number of patents in the name ofDuPont Canada Inc (e.g. U.S. Pat. No. 5,589,555, issued Dec. 31, 1996).

The catalyst components may be fed to a reaction either as a slurry orsolution in any one or a number of different hydrocarbons includingaromatic and non-aromatic hydrocarbons.

Other polymers that can be made in a solution polymerization processaccording to the current invention include but are not limited tohomopolymers, copolymers and terpolymers of one or more of propylene,styrene and butadiene.

Catalysts useful for solution polymerization are well known in the art.In general the invention can be used with any single site catalyst(SSC), Ziegler-Natta, chromium catalyst or any other organometalliccatalyst capable of polymerizing olefins in a solution process.

Single site catalysts generally contain a transition element of Groups3-10 of the Periodic Table and at least one supporting ligand. Somenon-limiting examples of single site catalysts include metalloceneswhich contain two functional cyclopentadienyl ligands (see for exampleWO 9941294), constrained geometry catalysts (see for example EP 418044)and catalysts having at least one phosphinimide ligand (see for exampleU.S. Pat. No. 6,777,509).

Single site catalysts are typically activated by suitable cocatalyticmaterials (i.e. “activators”) to perform the polymerization reaction.Suitable activators or cocatalytic materials are also well known tothose skilled in the art. For example, suitable cocatalysts include butare not limited to electrophilic boron based activators and ionicactivators, which are well known for use with metallocene catalysts,constrained geometry catalysts and catalysts having at least onephosphinimide ligand (see for example, U.S. Pat. No. 5,198,401 and U.S.Pat. No. 5,132,380). Suitable activators including boron basedactivators are further described in U.S. Pat. No. 6,777,509. In additionto electrophilic boron activators and ionic activators, alkylaluminum,alkyl/alkoxyaluminum, alkylaluminoxane, modified alkylaluminoxanecompounds and the like, can be added as cocatalytic components. Suchcomponents have been described previously in the art (see for exampleU.S. Pat. No. 6,777,509).

The term “Ziegler Natta catalyst” is well known to those skilled in theart and is used herein to convey its conventional meaning. Ziegler Nattacatalysts comprise at least one transition metal compound of atransition metal selected from groups 3, 4, or 5 of the Periodic Table(using IUPAC nomenclature) and an organoaluminum component that isdefined by the formula:Al(X′)_(a)(OR)_(b)(R)_(c)wherein: X′ is a halide (preferably chlorine); OR is an alkoxy oraryloxy group; R is a hydrocarbyl (preferably an alkyl having from 1 to10 carbon atoms); and a, b, or c are each 0, 1, 2, or 3 with theprovisos, a+b+c=3 and b+c>=1. As will be appreciated by those skilled inthe art of ethylene polymerization, conventional Ziegler Natta catalystsmay also incorporate additional components such as an electron donor.For example, an amine or a magnesium compound or a magnesium alkyl suchas butyl ethyl magnesium and a halide source (which is typically achloride such as tertiary butyl chloride). Such components, if employed,may be added to the other catalyst components prior to introduction tothe reactor or may be added directly to the reactor. The Ziegler Nattacatalyst may also be “tempered” (i.e. heat treated) prior to beingintroduced to the reactor (again, using techniques which are well knownto those skilled in the art and published in the literature).

The current invention also contemplates the use of chromium catalyststhat are also well known in the art. The term “chromium catalysts”describes olefin polymerization catalysts comprising a chromium species,such as silyl chromate, chromium oxide, or chromocene on a metal oxidesupport such as silica or alumina. Suitable cocatalysts for chromiumcatalysts, are well known in the art, and include for example,trialkylaluminum, alkylaluminoxane, dialkoxyalkylaluminum compounds andthe like.

In the current invention the term “polymer solution” can be any solutioncontaining both dissolved or molten homo- or co-polymer, one or moresolvents, one or more catalyst components, and one or more monomers orcomonomers. The polymer solution may also contain quenching reagents aswell as dissolved inert gases or dissolved hydrogen gas.

The effluent polymer solution from the reactor (or from the finalreactor if two or more reactors are used in series) is quenched byaddition of a catalyst deactivator and then heated by passage through aheat exchanger system. The “heat exchanger system” of the currentinvention comprises at least one heat exchanger. The catalystdeactivator suppresses or stops further polymerization within thepolymer solution. The heat exchanger effluent is fed to a flashdevolatilization system.

Catalyst deactivators can be used either to slow down the rate ofpolymerization or to stop it entirely. In a typical solution plant, thedeactivators used substantially stop the polymerization reaction bychanging the active catalyst into an inactive form. Most commonly, theseactivators take the form of polar molecules containing active hydrogenatoms and/or atoms which are capable of irreversibly binding to themetal center of the active catalyst.

In the current invention, those catalyst components that react with acatalyst deactivator are defined as “deactivator reactive catalystcomponents”. Deactivator reactive catalyst components may be single sitecatalysts, Ziegler-Natta catalysts, chromium catalysts, organometalliccatalysts, and cocatalysts or activator compounds, which irreversiblyreact with a catalyst deactivator.

Suitable deactivators are well known in the art and include but are notlimited to amines (see U.S. Pat. No. 4,803,259 to Zboril et al.); alkalior alkaline earth metal salts of carboxylic acid (see for example U.S.Pat. No. 4,105,609 to Machon et al); water (see for example U.S. Pat.No. 4,731,438 to Bernier et al); hydrotalcites (see U.S. Pat. No.4,379,882), alcohols and carboxylic acids (see U.S. Pat. No. 6,180,730).

Generally, the catalyst deactivators will be added in the minimum amountrequired to substantially quench the polymerization reaction. This inturn minimizes cost, but also the amount of un-reacted catalystdeactivators present in the product resin.

The use of one or more deactivators or mixtures thereof is alsocontemplated by the current invention.

Preferred deactivators should also satisfy the following requirements: adeactivator must deactivate the catalyst rapidly; should not deposit onthe equipment (particularly on heat exchanger surfaces); should notgenerate color or odor and should be safe and non-toxic. This limits thetypes of useful deactivators and dictates the way they are added to thereactor effluent. Accordingly, the selection of optimal deactivators andthe method of their use depends upon the polymerization process used.

In an embodiment of the current invention, each heat exchanger in a heatexchanger system will be a shell and tube type heat exchanger. Due totheir ease of operation and to their good reliability, shell and tubeheat type heat exchangers have become a preferred means for cooling orheating fluid flows and are well known in the art. However, other heatexchangers including but not limited to double pipe, flat-plate andspiral wound heat exchangers may also be used with the currentinvention.

In a heat exchanger, a “hot” fluid is brought into thermal contact witha “cold” fluid facilitating heat transfer across a heat exchangesurface. The heat transfer may occur by a forced convection or acondensation mechanism. The heat exchange surface is preferablyconstructed from stainless steel or from other suitable metal alloysthat allow for good heat transfer. A typical shell and tube heatexchanger is comprised of an open shell like structure, the “shell side”which encases a number of tubes that traverse the length of the shellstructure. The inside of the tubes is collectively designated as the“tube side”. The tubes are held in a “tube sheet” at each end of theshell housing, the tube sheet forming a barrier between the tubechannels and the inside of the shell. The shell housing is equipped withan inlet and an outlet, between which a series of baffles may be placedto provide a torturous fluid flow pathway. During heat exchange, a fluidflow traverses the distance from inlet to outlet on the “shell side” ofthe exchanger, and comes into thermal contact with a fluid flowtraversing the length of the tubes on the “tube side” of the exchanger.The “tube side” of a heat exchanger can also be said to have an inletand an outlet upstream and downstream of each end of the tube sheetrespectively.

In a preferred embodiment of the current invention a “hot” fluid flowsthough a heat exchanger on the “shell side”, while a polymer solution,which is the “cold” fluid, passes through the heat exchanger on the“tube side”. The heating fluid may be, but is not limited to, steam,oil, or recycled monomer/solvent streams. Without wishing to be bound bytheory, the heat transfer may occur via a forced convention orcondensation mechanism. In a preferred embodiment of the currentinvention, the heating fluid is high pressure steam.

Increasing the number of tubes that are enclosed within the shellstructure increases the overall surface area of the heat exchangesurface between the two fluids. In a preferred embodiment, the number oftubes is sufficient to provide efficient heat transfer to a polymersolution passing through the heat exchanger at a mass flow rate of morethan about 180-400 tons/h. In one embodiment, the tubes may be 0.5 to1.0 inch in diameter and there may be over 3000 tubes in a heatexchanger tube sheet. In another embodiment, static mixing inserts canbe added to the tubes to improve polymer solution mixing and heattransfer efficiency.

Heat exchangers used in the current invention may be of a single-pass ora multi-pass type, examples of which are well known in the art. The flowof fluids through the heat exchanger may be cross or counter flow (flowof “hot” and “cold” fluids is in approximately opposite directions) oruni-direction parallel flow (“hot” and “cold” fluids flow inapproximately the same direction). The fluid on the shell side may alsocondense to yield efficient heat transfer. In a preferred embodiment, aone pass shell and tube heat exchanger is operated in a cross-counterflow arrangement. Heat exchangers may be configured vertically orhorizontally, but are preferably in a vertical configuration.

In the current invention, heat exchangers will have a “tube side”polymer solution inlet temperature, T_(cold,inlet), (i.e. thetemperature at the front end or upstream end of the heat exchanger tubesheet) and a “tube side” polymer solution outlet temperature,T_(cold,outlet), (i.e. the temperature at the back end or the downstreamend of the heat exchanger tube sheet), since the polymer solution is the“cold” fluid and flows through the tube side of the heat exchanger. Thepolymer solution flowing through a heat exchanger will have atemperature that increases along a temperature gradient as it flows frominlet to outlet. For a heat exchanger immediately downstream of thereactor system, the T_(cold,inlet) will be similar to the temperature inthe final polymerization reactor and can be in the range of betweenabout 150° C. and 230° C. Similarly, the heat exchangers will have a“shell side” heating fluid (i.e. the “hot” fluid) inlet temperature,T_(hot,inlet), and a “shell side” heating fluid outlet temperature,T_(hot,outlet). The T_(cold,inlet), T_(hot,inlet), T_(hot,outlet), andT_(cold,outlet) can be determined by any well known method, such as butnot limited to the use of a thermocouple device.

In an embodiment of the invention, the heat exchanger system comprises asingle heat exchanger, provided sufficient heat is transferred to thepolymer solution for efficient devolatilization. In another embodimentof the current invention, the heat exchanger system comprises acombination of heat exchangers, individually in series or in parallel,to achieve sufficient heat transfer to the polymer solution forefficient devolatilization. Sufficient temperatures are from about 220°C. to 300° C. (i.e. T_(cold,outlet) of the final heat exchangerimmediately upstream of the devolatilizer is from 220° C. to 300° C.).In another aspect of the invention the T_(cold,outlet) will be from 250°C. to 300° C.

A heat exchanger will have a “tube side” inlet pressure, P_(IN) (i.e.the pressure at the front end or upstream end of the heat exchanger tubesheet) and a “tube side” outlet pressure, P_(OUT) (i.e. the pressure atthe back end or downstream end of the heat exchanger tube sheet). In thecurrent invention, P_(IN) will be greater than P_(OUT) such thatP_(IN)−P_(OUT) is positive. P_(IN) and P_(OUT) are readily measuredusing any well known technique, such as but not limited to transducersor pressure gauges. Convenient units for P_(IN) and P_(OUT) are MPag.

The heat exchanger inlet and outlet pressures can be adjusted usingpressure let down valves that are upstream and downstream of a heatexchanger respectively. If two or more heat exchangers are used inseries then additional pressure let down valves may be incorporated suchthat there is at least one let down pressure valve between each of theheat exchangers.

In an embodiment of the current invention, two or more heat exchangersare used in series, each of which is downstream of a finalpolymerization reactor, and upstream of a flash devolatilization system.

The efficiency of heat transfer within a shell-and-tube heat exchangerwith one or more tube passes is expressed using the overall heattransfer coefficient, U. The overall heat transfer coefficient, U, foreach heat exchanger is related to the total rate of heat transferred inthat heat exchanger, Q. The total rate of heat transfer, Q, can bedetermined from the temperature rise of the fluid on the tube side ofthe heat exchanger or from the loss of enthalpy from the heating fluidon the shell side of the heat exchanger using the following equation,Q=FUAΔT_(lm)  (Eqn. 1)where F is a heat exchanger design factor, which in the currentinvention has been set to equal one (the F design factor is acharacteristic of a given heat exchanger design and is a dimensionlessquantity often equal to about one), A is the area available for the heattransfer through the tube walls, and ΔT_(lm) is the log mean temperaturedifference across the heat exchanger. The log mean temperaturedifference is a function of inlet and outlet temperatures on the colderand hotter sides of the heat exchanger respectively,

$\begin{matrix}{{\Delta\; T_{lm}} = \frac{{\Delta\; T_{1}} - {\Delta\; T_{2}}}{\ln\left( {\Delta\;{T_{1}/\Delta}\; T_{2}} \right)}} & \left( {{Eqn}.\mspace{14mu} 2} \right)\end{matrix}$where ΔT₁=T_(hot,inlet)−T_(cold,outlet) andΔT₂=T_(hot,outlet)−T_(cold,inlet) for a cross-counter flow heatexchanger. In the present invention, the polymer solution is the “cold”fluid flowing through the tube side of the heat exchanger. Hence, at theupstream side of the heat exchanger, the tube side inlet temperature isdefined as the T_(cold,inlet). The “hot fluid” flowing through the shellside has an inlet temperature, defined as the T_(hot,inlet). Similarly,the downstream end of the heat exchanger has a tube side,T_(cold,outlet) and a shell side T_(hot,outlet). For a given heatexchanger, A is known. The values of Q and ΔT_(lm) are determined fromthe process conditions. The value of Q is determined from the enthalpychange, either of the tube side fluid or of the shell side fluid. Theenthalpy change can be determined by integrating the heat capacity of afluid, when there is no phase change of the fluid, or from the enthalpyof condensation of the fluid when there is condensation. Equation 1 isthen used to calculate the value of the heat transfer coefficient, U,under a given set of process conditions. The more efficient a heatexchanger with a given heat exchange surface area is, the larger thevalue of the heat transfer coefficient, U. Equation 1 can be used tocharacterize heat transfer rates, and hence the coefficient U, in heatexchangers conveying single or multiphase fluids. U is convenientlyexpressed in kW/m²/K.Inducing Phase Separation

In a first aspect the current invention, the pressure of the singlephase polymer solution is reduced downstream of a reactor system to apressure that induces the polymer solution to phase separate into a twophase liquid-liquid polymer solution. Preferably, the phase separationoccurs downstream of a reactor system and upstream of a heat exchangersystem. Optionally, the phase separation may occur within a heatexchanger system. The pressure can be reduced downstream of the reactorsystem by use of one or more pressure let down valves located downstreamof a reactor system. The pressure let down valves can be upstream and/ordownstream of each heat exchanger.

In the current invention, a single phase (i.e. a single liquid phase)polymerization solution is preferably present in the reactor system, andprocess conditions, such as but not limited to monomer concentration,temperature and pressure, are controlled to avoid liquid-liquid phaseseparation in the reactor system.

The term “two-phase liquid-liquid polymer solution” is meant toencompass any polymer solution that comprises a distinct polymer richphase and a distinct polymer lean phase. A “polymer lean” phase isdefined as having at least 90 weight percent (wt %) of solvent. A“polymer-rich” phase is defined as having at least 10 weight percent (wt%) of polymer. In the current invention, concentration is typicallyexpressed in terms of weight fraction, wt % or weight percent, wt % of acomponent in a polymer solution.

Phase separation behavior and more specifically, cloud pointdetermination of polymer solutions can be modeled off-line usingrepresentative polymer solutions of known composition. In the currentinvention, phase separation data in the form of a vapor-liquid-liquid(VLL) diagram is obtained using a multi-pass rheometer (MPR). Themulti-pass rheometer is a capillary rheometer in which severalcapillaries of different lengths and diameters are enclosed within ahigh temperature/pressure cell that is capable of confining a solution.The vertical cell has pistons at both ends of the cell and duringoperation the solution is sheared back and forth through thecapillaries. Under conditions in which a steady shear is achieved, thepressure drop across the capillaries, P_(IN)−P_(OUT) is measured and theapparent viscosity of the fluid is determined as a function of the shearrate in each capillary. Before the shearing is initiated, the pistonsare moved with respect to each other in order to obtain a desired staticpressure for the system.

The “apparent viscosity”, μ_(A) of a polymer solution is defined for agiven shear rate as the pressure drop, ΔP across the capillaries when apolymer solution is forced through a capillary of constant diameter, ata constant static pressure, temperature and polymer solutioncomposition. The apparent viscosity, μ_(A) is equal to the ratio of“shear wall stress”, τ_(W), to “wall shear rate”, {dot over (γ)}_(WN),for a Newtonian fluid:

$\begin{matrix}\begin{matrix}{\mu_{A} = \frac{\tau_{W}}{{\overset{.}{\gamma}}_{WN}}} \\{= \frac{R^{2}\Delta\; P}{8{LV}}}\end{matrix} & \left( {{Eqn}.\mspace{14mu} 3} \right)\end{matrix}$where V is the average fluid velocity in the tube in m/s (i.e. thevelocity at which the pistons are moved within the capillaries), ΔP isthe pressure drop across the capillaries in MPa (or Pa), L is the lengthof the capillaries in meters, R is the radius of the capillaries inmeters and τ_(W) is defined as above.

A person skilled in the art will recognize that use of a multi-passrheometer in the prescribed manner allows for the measurement of theapparent viscosity of representative off line polymer solutions as afunction of temperature, pressure and shear rate.

The apparent viscosity of a polymer solution undergoes a dramatic changeat the cloud point pressure, due to the formation of two liquid phases.Measurement of the cloud point pressure at different temperatures yieldsa cloud point curve which demarcates the two liquid region from thesingle liquid region of the phase diagram for a given polymer solution.By way of example, the phase behavior of a polymer solution of thecurrent invention may be approximated by the isoplethic phase diagram(i.e. a vapor-liquid-liquid phase diagram) for a polymer-solvent mixturecontaining a small amount of monomer as provided in FIG. 1.

With reference to FIG. 1, A defines the liquid-amorphous solid region; Bdefines the single liquid region (i.e. the single phase polymersolution); C defines the liquid-liquid region (i.e. the two phaseliquid-liquid polymer solution); D defines the supercritical fluidregion; E defines the vapor-liquid-liquid region; F defines thevapor-liquid region; 2 defines the cloud point boundary or the lowercritical solution temperature (the LCST) curve; 1 defines the lowercritical end point; 3 defines the critical temperature of solvent; and 4defines solvent vapor-liquid critical point. For the polymer solutionsof the current invention, 5 a is the lower boundary of thevapor-liquid-liquid region and 5 b is the upper boundary of thevapor-liquid-liquid region.

It will be recognized by one skilled in the art, that the pressurerequired for inducing the formation of two liquid phases for a range ofpolymer solutions can be predicted by generating a curve similar to thatshown in FIG. 1. Hence, generation of vapor-liquid-liquid orliquid-liquid phase diagrams for representative polymer solutions allowsfor the prediction of on-line conditions under which liquid-liquid phaseseparation occurs.

A two-phase polymer solution of the current invention may undergo one ormore phase inversions during flow through a heat exchanger.

Without wishing to be bound by theory, phase-inversion behavior may leadto rapid changes in the apparent viscosity of the two-phase polymersolution. By phase inversion, it is meant that the polymer solutioninterconverts between a system comprising polymer lean phase dropletsdispersed in a continuous polymer rich phase and a system comprisingpolymer rich phase droplets dispersed in a continuous polymer leanphase.

The exact location of the liquid-liquid phase boundary in the presentinvention (i.e. pressure required for obtaining polymer solution phaseseparation) will depend on a number of other factors (in addition totemperature and pressure) including but not limited the weight fractionsof polymer, solvent or monomer dissolved in the polymer solution and thepolymer molecular weight distribution. As a result, there is no one setof conditions under which phase separation may be induced.

For the purposes of this invention, factors such as the temperature ofthe polymer solution, the weight fraction of monomer dissolved in thepolymer solution, the weight fraction of polymer in the polymersolution, the polymer molecular weight and the solvent composition aresuch that the liquid-liquid phase boundary is traversable undercommercially viable pressures.

Surface Active Agents

In a second aspect of the present invention, at least one surface activeagent is added to the polymer solution downstream of a reactor systemand upstream of a heat exchanger system.

The surface active agent may be added in any known manner. By way ofnon-limiting example, the surface active agent may be added as asolution in the main process solvent and introduced into the flow at atee junction, following which it is mixed into the main flow using astatic mixing element in the main pipe.

The addition of a surface active agent is used in combination withconditions under which the polymer solution is induced to undergo phaseseparation into a two phase liquid-liquid polymer solution. A surfaceactive agent can be added to the polymer solution before or after thepolymer solution is induced to undergo liquid-liquid phase separation,provided that the surface active agent is added upstream of a heatexchanger system.

In the current invention, when a two phase liquid-liquid polymersolution is formed, the presence of sufficient amounts of surface activeagent increases the heat transfer coefficient, U, of at least one heatexchanger in a heat exchanger system, thereby increasing the efficiencyof heat transfer from a heat transfer fluid (i.e. the “hot” fluid) tothe polymer solution (i.e. the “cold” fluid”).

In an embodiment of the invention, the surface active agents will beadded in amounts from 0.1 ppm to 1000 ppm, preferably from 1 ppm to 1000ppm, even more preferably from 1 ppm to 100 ppm.

By adding a surface active agent, the heat transfer coefficient, U canbe increased by at least 5%, preferably by at least 10%.

Without wishing to be bound by theory, the low levels of surface activeagent may collect at the interfaces between the liquid interfaces of atwo phase liquid-liquid polymer solution in such a way as to enhance theefficiency of heat transfer from a heat transfer fluid, across a heatexchange surface, to the two phase liquid-liquid polymer solution. Forexample, the surface active agent may lower the interfacial tension, bypromoting phase inversion of polymer lean and polymer rich phases, to alevel that permits the stabilization of a two-phase morphology, which isconducive to efficient heat transfer. Reducing the interfacial tensionthen between polymer lean and polymer rich liquid phases, may facilitatethe coating of a heat transfer surface by a phase which has high thermalconductivity, or alternatively, by the stabilization of a droplet sizedistribution which leads to a more effective thermal conductivity.

The surface active agents used in the current invention can be selectedfrom a wide range compounds comprising a hydrophobic tail moiety and ahydrophilic head moiety. Such compounds include amphiphilic anionic,cationic and neutral compounds, as well as amphoteric compounds, all ofwhich are well known in the art. Anionic surface active agents arecompounds in which the hydrophilic head moiety has an anionic charge ormay assume an anionic charge in aqueous solution. Cationic surfaceactive agents are compounds in which the hydrophilic head moiety has acationic charge or may assume a cationic charge in aqueous solution.Non-ionic surface active agents are compounds that do not dissociateinto charged ions in aqueous solution. Amphoteric surface active agentshave a hydrophilic head moiety that is zwitterionic (i.e. has both ananionic and a cationic charge).

In an embodiment of the current invention, the anionic surface activeagents are selected from the group consisting of carboxylate, sulfate,sulfonate, phosphate, phosphonate, compounds and mixtures thereof. Thecarboxylate, sulfate, sulfonate, phosphate and phosphonate, compoundsmay be used in acid or salt form. Phenolate; cyanate; and thiocyanatecompounds; as well as polyelectrolytes and anionic polymers such as butnot limited to polyacrylate are also contemplated for use with thecurrent invention.

In an embodiment of the invention, anionic surface active agentsinclude, but are not limited to carboxylic acids; sulfonic acids;sulfuric acids esters; phosphoric acid esters; phosphonates; and saltsthereof, bearing alkyl, aryl, aralkyl, or alkaryl groups having from sixto thirty carbon atoms (i.e. C₇ to C₃₁ carboxylic acids, sulfuric acidesters, sulfonic acids, phosphonic acids, phosphoric acid esters and/orsalts thereof). The alkyl groups may be linear or branched, saturated orunsaturated.

In an embodiment of the invention, anionic surface active agentsinclude, but are not limited to salts or acids of: carboxylates such aslauryl, stearyl, oleyl and cetyl carboxylates; sulfates such as alkylether sulfates, alkyl ester sulfates and alkyl benzene sulfates;sulfonates such as alkylbenzene sulfonate, alkylnaphthalene sulfonate,paraffin sulfonate; phosphonates; phosphates such as alkyl etherphosphates or alkyl ester phosphates, and polyphosphates.

In an embodiment of the current invention, the anionic surface activeagents are salts or acids of: carboxylates, (R⁵)COO⁻; phosphates,(R⁵)OPO(OH)O⁻; sulfates, (R⁵)OSO₃ ⁻; sulfonates, (R⁵)SO₃ ⁻ and mixturesthereof where, R⁵ is selected from the group consisting of linear orbranched, saturated or unsaturated alkyl groups having from 3 to 30carbon atoms; aralkyl groups which are substituted benzyl moietiesincluding fused ring moieties, having linear chains or branches of from3 to 22 carbons; alkaryl or substituted aryl groups including fused ringgroups, having linear chains or branches of from 3 to 22 carbons.

In an embodiment of the current invention, the surface active agent is acarboxylic acid having a linear saturated alkyl group that is 5 to 13carbons long (i.e. a C₆ to C₁₄ carboxylic acid) or a mixture of at leasttwo carboxylic acids having a linear saturated alkyl group that is 5 to13 carbons long.

In an embodiment of the current invention, the surface active agent is acarboxylic acid having a linear saturated alkyl group that is 6, 8, 10or 12 carbons long (i.e. a C₇, C₉, C₁₁ or C₁₃ carboxylic acid) or amixture thereof, such as but not limited to pelargonic acid and nonanoicacid.

In an embodiment of the invention, the cationic surface active agentsare selected from the group consisting of quaternary ammonium,phosphonium, sulfonium, pyridinium, and imidazolium compounds andmixtures thereof.

In an embodiment of the invention, the cationic surface active agent isa compound that has at least one long chain linear or branched,saturated or unsaturated alkyl group having from 6 to 30 carbon atoms.The remaining groups of the cation can be selected from the groupconsisting of hydrogen; linear or branched, saturated or unsaturatedalkyl groups; cyclic alkyl groups; aromatic groups; benzyl groups andsubstituted benzyl groups; and the like. Suitable long chain alkylgroups may be derived from naturally occurring oils, animal oils orfats, or may be petrochemically derived. Some non-limiting examplesinclude methylstearyl, ethylstearyl, methyloleyl, ethyloleyl, layryl,stearyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, hydrogenatedtallow, docosanyl, oleyl, linoleyl, linolenyl, soya and tallow.

In an embodiment of the invention, the cationic surface active agentcontains at least one linear or branched alkyl, aliphatic, aralkyl,alkaryl, or aromatic hydrocarbon group having from 6 to 30 carbon atoms,or alkyl or alkyl-ester groups having from 6 to 30 carbon atoms. Theremaining groups of the cationic surface active agent can be selectedfrom a group consisting of linear or branched alkyl groups containingfrom 1 to 30 carbon atoms; aralkyl groups such as benzyl and substitutedbenzyl moieties including fused ring moieties, having linear chains orbranches of from 1 to 22 carbons; alkaryl groups; aryl groups such asphenyl and substituted phenyls including fused ring aromatic groups andsubstituents; and hydrogen.

In an embodiment of the current invention, the cationic surface activeagent can be a salt of: [(R¹)(R²)(R³)(R⁴)N]⁺, [(R¹)(R²)(R³)(R⁴)P]⁺,[(R¹)(R²)(R³)S]⁺ or mixtures thereof, where R¹ is a linear or branchedalkyl, aralkyl, alkaryl, or aromatic hydrocarbon group having from 8 to30 carbon atoms, or alkyl or alkyl-ester groups having from 8 to 30carbon atoms; and R² to R⁴ are selected from the group consisting oflinear or branched alkyl groups containing from 1 to 30 carbon atoms;aralkyl groups such as benzyl and substituted benzyl moieties includingfused ring moieties, having linear chains or branches of from 1 to 22carbons; alkaryl groups; aryl groups such as phenyl and substitutedphenyls groups including fused ring aromatic groups and substituents;and hydrogen.

In an embodiment of the current invention, quaternary ammonium orphosphonium compounds bearing alkyl, aryl, aralkyl or alkaryl groups areused as surface active agents.

In an embodiment of the invention, the non-ionic surface active agentsare selected from the group consisting of ethoxylates, polyethyleneglycol ethers, fatty acid alkylolamides, sucrose fatty acid esters,polyglucosides, or amine oxides and mixtures thereof. Suitable non-ionicsurface active agents include but are not limited to C₆-C₁₈ alkylphenolalkoxylates, C₁₂-C₂₀ alkanol alkoxylates and block copolymers ofethylene oxide and propylene oxide, and C₄-C₁₈ alkyl glucosides, andproducts obtained by reaction of alkyl glucosides with ethylene oxide.

In an embodiment of the invention, the amphoteric surface active agentsare selected from the group consisting of aminocarboxylic acids,betaines, and sulfobetaines and mixtures thereof.

The surface active agent can be added to the polymer solution before orafter the addition of a catalyst deactivator or at the same time,provided that the addition of the surface active agent is upstream of aheat exchanger system. A surface active agent may be added to thepolymer solution upstream or downstream of a catalyst deactivator or atthe same entry point, provided that the addition of the surface activeagent is upstream of a heat exchanger system.

In some embodiments of the invention, the surface active agentsdescribed above, are also capable of quenching the polymerizationreaction. For example, surface active agents containing an activehydrogen such as but not limited to carboxylic acids, sulfuric acidesters, sulfonic acids, phosphoric acid esters and phosphonic acids maybe used to quench the polymerization in addition to increasing thecoefficient of heat transfer in at least one heat exchanger. Thus, insome embodiments of the invention, the catalyst deactivator and thesurface active agent can be the same compound and separate process stepsto introduce the catalyst deactivator and the surface active agent arenot required.

In an embodiment of the current invention, the surface active agent isalso a catalyst deactivator and is added to a polymer solutiondownstream of a reactor system and upstream of a heat exchanger system.

Where the surface active agent also acts as a catalyst deactivator, theamount of surface active agent required to obtain an increase in theheat exchanger system efficiency (as measured by an increase in the heattransfer coefficient, U of at least one heat exchanger) must besufficient to overcome the amount of surface active agent consumedduring reactions to quench polymerization. By way of non-limitingexample, less than about 3 ppm of surface active agent is required toquench the polymerization reaction, and the amount of surface activeagent required to increase the heat exchange coefficient, U is more thanabout 3 ppm, for example, from about 3 to 15 ppm.

Further details of the invention are illustrated by the followingnon-limiting example.

EXAMPLES

FIG. 2 displays the results of heat transfer experiments conducted at apilot plant scale under two different pressure regimes in the presenceof a surface active agent. In the experiments, a six-pass shell and tubeheat exchanger was used to heat a reactor effluent polymer solutioncontaining approximately 15 wt % polymer in a hydrocarbon solventcontaining residual ethylene. The flow rate of polymer solution throughthe heat exchanger was approximately 750 kg/h. The surface active agentused was a mixture of carboxylic acids, mainly aliphatic carboxylicacids having from 6 to 9 carbons. The two curves in FIG. 2 representdata obtained for a given polymer solution at a pressure in which asingle liquid phase polymer solution is present (15 MPag), and at apressure under which phase separation has occurred (8 MPag) to provide atwo phase liquid-liquid polymer solution (i.e. at 8 MPag the pressure isbelow the phase separation or cloud point pressure). The pressures inFIG. 2 correspond to the heat exchanger tube side inlet pressure,P_(IN).

The data shows, that for a liquid-liquid, two phase polymer solution,increasing the amount of surface active agent, which is represented inFIG. 2 as an increase in the molar ratio of surface active agent todeactivator reactive catalyst components of from 0.5 to 1.5, results inan enhancement of the heat transfer coefficient, U, of from 0.27 kW/m²/Kto 0.4 kW/m²/K, an increase of 48%. Increasing the amounts of surfaceactive agent to levels beyond those shown on this plot did not result inany further significant enhancement in the heat transfer coefficient, U,demonstrating that the effect of the surface active agent on the heattransfer coefficient reaches a maximum (above a ratio of about 1.5).

For a single phase polymer solution (at 15 MPag, which is above thephase separation or cloud point pressure of the polymer solution) theconcentration of the surface active agent had no effect on the heattransfer coefficient, U (the lower line in FIG. 2).

With reference to FIG. 2, the effect of the inventive process on theheat transfer coefficient, U, is fully reversible. If the addition ofsurface active agent is discontinued at a constant pressure of 8 MPag,then the U value returned to approximately its original value.Similarly, if at a constant level of surface active agent, the originaloperating pressure is restored to provide a single phase polymersolution (i.e. 15 Mpag), the U value is likewise restored (i.e. Udecreased to approximately its original value). Thus it will berecognized by the person skilled in the art, that the surface activeagent is not having a bulk effect, such as cleaning the heat exchangesurface in a heat exchanger.

In these experiments, the surface active agent also acted as a catalystdeactivator, and a molar ratio of surface active agent to deactivatorreactive catalyst components of about at least 0.3 was required toquench the polymerization reaction. The surface active agent consumed inthe catalyst quenching reactions, did not increase the heat transfercoefficient, U.

The present invention, is not meant to be limited to any particularscale or process and is useful over a wide range of polymer solutionflow rates through a heat exchanger system, including commerciallyrelevant flow rates for a commercial scale LLDPE solution polymerizationplant. By way of a non-limiting example, the flow rate of polymersolution through the heat exchanger system can be from about 300 kg/h toabout 500,000 kg/h.

1. A high temperature solution polymerization process comprising: (a)polymerizing one or more than one olefin in a solvent within a reactorsystem to produce a single phase polymer solution; (b) quenching thepolymerization reaction downstream of the reactor system with a catalystdeactivator; (c) reducing the pressure of the single phase polymersolution downstream of the reactor system to a pressure which inducesthe single phase polymer solution to undergo liquid-liquid phaseseparation into a two phase polymer solution; and (d) feeding the twophase polymer solution through a heat exchanger system comprising atleast one heat exchanger, wherein the heat transfer coefficient, U, ofat least one heat exchanger is increased by adding at least one surfaceactive agent to the polymer solution downstream of the reactor systemand upstream of the heat exchanger system.
 2. The process of claim 1,wherein the surface active agent is selected from the group consistingof carboxylate, sulfate, phosphate, phosphonate, and sulfonate compoundscomprising from 6 to 30 carbon atoms.
 3. The process of claim 2, whereinthe surface active agent is a carboxylic acid comprising a linear orbranched alkyl group comprising from 6 to 30 carbon atoms.
 4. Theprocess of claim 3, wherein the pressure is reduced by opening one ormore pressure let down valves.
 5. The process according to claim 4,wherein the surface active agent is added in amounts of from 1 to 1000parts per million.
 6. The process according to claim 5, wherein thesurface active agent increases the heat transfer coefficient, U of atleast one heat exchanger by at least 10%.
 7. The process according toclaim 6, wherein the olefin comprises ethylene and optionally acomonomer.
 8. The process according to claim 7, wherein the heatexchanger system comprises one or more shell and tube heat exchangers.9. The process according to claim 8, wherein the reactor system is astirred tank reactor system.
 10. The process according to any one ofclaims 1-9, wherein the surface active agent and the catalystdeactivator are the same compound, provided that the surface activeagent is added in an amount higher than the amount of surface activeagent required to quench the polymerization reaction.